1. Field of the Invention
The present invention relates to a hard disk drive and, more particularly, to a disk drive and a method adapted to determine a signal width providing an optimal off-track position for a magnetoresistive head, and to a magnetic disk having a servo burst with such a signal width written in a specific area thereupon. The invention enables the track density of the disk to be raised by using the servo burst during track following operations.
2. Description of the Related Art
Generally, a hard disk drive magnetically writes data onto and reads data from a rotating magnetic disk. Such disk drives are widely used as auxiliary memory devices in computer systems because they can access large amounts of data at high speeds. A hard disk drive typically stores data in several substantially circular tracks arranged concentrically on a surface of the disk. The disk drive includes at least one magnetic head that accesses these tracks to read data from and write data to the magnetic disk. Heretofore, the heads used in disk drives have mainly been magnetic inductive heads such as Metal In Gap (MIG) heads or thin film heads of a standard ring type.
Magnetic inductive head technology has certain limitations that have increasingly caused the disk drive industry to move toward the use of magnetoresistive heads. Unlike magnetic inductive heads, the performance of magnetoresistive heads depends only slightly upon the time rate of change of magnetic flux between polarized regions on the disk surface. This relative insensitivity to flux change rate allows a magnetoresistive head to perform acceptably when the rotational speed of the disk is increased. A magnetoresistive head therefore can perform well with a disk having greatly increased linear density of flux change (measured in units of Flux Change Per Inch, or FCPI) which is linear density of flux change in the track direction (i.e., the circumferential direction).
Despite this advantage, certain limitations of magnetoresistive head technology have delayed the widespread use of magnetoresistive heads. One particularly serious limitation has been that, due to their structural features, magnetoresistive heads require a radial track density (measured in Tracks Per Inch, or TPI, which represents the spacing between tracks in the radial direction) relatively lower than that required for thin film heads. These structural features create a track shift as between the reading portion of the head and the writing portion of the head. Unless the disk drive successfully compensates for it, this track shift reduces the maximum (radial) track density that can be accommodated by the magnetoresistive head.
The track shift inherent in a magnetoresistive head exists because of a difference between the effective radial position of a given track with respect to the read portion and the effective position with respect to the write portion. Such a difference is significant, of course, because the magnetoresistive head, in addition to reading user data from the disk, also desirably reads servo data used for determining the position of the head with respect to a desired track. The existence of the track shift means that, when the read portion of the head is accurately centered over the track, the write portion of the head is positioned away from the track center.
This off-track position usually deviates only slightly from the track center, but the amount of deviation nevertheless significantly affects the maximum track density that the head can accommodate. With even a slight off-track deviation, the inter-track spacing must be commensurately increased to prevent a write operation with respect to the given track from interfering with data recorded on an adjacent track. Because increasing the inter-track spacing reduces the track density, compensating for the off-track deviation of the write portion of the head has become a high priority in efforts to use magnetoresistive heads in conjunction with high-density disks.
Various ingenious approaches have been suggested to address this problem. For example, U.S. Pat. No. 5,615,063, entitled "Magnetoresistive Head Bias Current Switching Based On Skew Angle" and issued Mar. 25, 1997 to Kuroki et al., the disclosure of which is incorporated herein by reference, discloses an off-track compensation system using current reversal in the read portion of the magnetoresistive head. This current reversal causes the head to detect an effective servo signal that partly compensates for the track shift between the read portion and the write portion of the head.
U.S. Pat. No. 5,596,463, entitled "Recording/Reproduction Apparatus With An Integrated Inductive Write, Magnetoresistive Read Head" and issued Jan. 21, 1997 to Hashimoto, the disclosure of which is incorporated herein by reference, provides another approach for addressing the track shift problem. This patent shows a mechanical compensator that adjusts the orientation of the magnetoresistive head with respect to the actuator as the head moves radially along the disk. By turning the head in this manner, at least some of the variation in track shift between tracks is compensated.
A possible alternative to electrical and mechanical off-track compensation systems is to intentionally place the read portion of the magnetoresistive head in an off-track position selected to center the write portion of the head over the track to be written. The difficulty with such an approach resides in selecting the off-track position for the read head. To effectively compensate for the intrinsic off-track of the head, a track shift should be predicted for each track individually.
Theoretically, this prediction could be carried out by a controller of the disk drive in accordance with a mathematical model of the variation of track shift with radial position of the track. In practice, though, such computational prediction it is not easy to carry out with sufficient accuracy because the magnitude of track shifts varies with the structure of the head and numerous other factors that differ between individual disk drive units. Prediction of track shifts relative to uncompensated servo burst signals also can result in reduced sensitivity of servo control if the track shifts are large.
What has been needed, and what seemingly has not yet been found, is a compensation approach that uses the disk itself to provide off-track compensation information. Some efforts to compensate for other types of off-track error have used servo signals written to the disk. For example, U.S. Pat. No. 5,606,469 shows a method that uses an additional servo pattern written to a data disk to compensate for an apparent off-track condition that arises when a servo head reading a dedicated servo surface and a data head reading servo information from the data disk fall out of calibration.
The objective of the '469 patent, though, is to ensure that the data head reads from the center of the data track as defined by track-centering servo burst patterns. It provides a servo burst pattern that enables the read transducer to be centered on a given track despite the fact that the transducer incorrectly reads the standard servo patterns provided for track centering. The disclosed system therefore operates in a manner opposite to the desired operation for compensation of track shifts associated with a magnetoresistive head. In fact, the disclosure of this patent does not address magnetoresistive heads or the special off-track problems that arise with them.
U.S. Pat. No. 5,339,207 also uses an additional signal field in a servo sector of a disk, but here the objective is to determine the head transducer gain and to measure the width of the read transducer during calibration of the disk drive. A method is disclosed for writing a filler pattern in the servo sector following a standard pattern of servo bursts. This filler pattern is uniform for all odd-numbered tracks and for all even-numbered tracks, and thus it cannot provide off-track information for compensation of track shifts associated with a magnetoresistive head. In fact, as with the '469 patent, the disclosure of this patent does not address magnetoresistive heads or the special off-track compensation problems associated with them.
I have therefore found that a continuing and unmet need exists for an effective and efficient approach to compensation for the off-track deviation caused by track shifts in magnetoresistive heads. An invention realizing such an approach would provide rapid and accurate compensation of track shifts appropriate for the particular components of the disk drive and would not require model-based prediction calculations. Such an invention also should not rely upon electrical or mechanical techniques whose precision could suffer under adverse environmental conditions or as the apparatus ages. Preferably, the invention could be implemented with few or no hardware changes to existing designs for disk drive servo control systems.